You are technically correct that processed foods are no less healthy than any other foods.

However they are engineered to be hyper palatable, meaning they taste way better than other foods and they will not satiate you even after eating and excessive amount.

Or to put it simply, it is really damn easy to eat a thousand calories of potato chips and still be hungry for "a real meal" but most people will tap out before eating a thousand calories of steak (12 to 16oz).

I think focus on satiation & calorie density makes good sense.

"Ultra-processed" is not completely precise, not completely accurate proxy for "hyper palatable, low satiation, calorie dense".